1. Field of the Invention
The present invention relates to wiring formed by using a thin film technique and a method of manufacturing the same. The present invention also relates to a wiring board and a method of manufacturing the same. In the present specification, a wiring board refers to an insulating substrate made of glass, etc. or various substrates having wiring formed by using a thin film technique.
2. Description of the Related Art
In recent years, a technique of forming a thin film transistor (TFT) using a semiconductor thin film (thickness of about several to hundreds of nm) formed on a substrate with an insulating surface has been paid attention to. A TFT is widely applied to an electronic device such as an integrated circuit (IC) and an electrooptic apparatus. In particular, a TFT is being rapidly developed as a switching element of an image display apparatus.
Conventionally, a liquid crystal display device as an image display apparatus is known. Since an image of higher precision can be obtained compared with a passive type liquid crystal display device, an active matrix type liquid crystal display device is being used more. In an active matrix type liquid crystal display device, pixel electrodes arranged in matrix are driven, whereby a display pattern is formed on a screen. More specifically, a voltage is applied between a selected pixel electrode and a counter electrode corresponding to the pixel electrode whereby a liquid crystal layer disposed between the pixel electrode and the counter electrode is subjected to optical modulation, and this optical modulation is recognized by an observer as a display pattern.
Such an active matrix type liquid crystal display device finds a wider range of uses, and there is an increasing demand for high precision, a high aperture ratio, and high reliability, as well as enlargement of a screen size. There is also a demand for enhancement of productivity and a decrease in cost.
In the case where aluminum (Al) is used as wiring of the above-mentioned TFT in order to manufacture TFT, projections such as hillock and whisker are formed due to a heat treatment, and an Al atom diffuses to an insulating film and an active region (in particular, a channel-formation region), which may cause operation defects of the TFT or a decrease in electrical characteristics of the TFT.
Under such a circumstance, the use of a metal material withstanding a heat treatment (typically, a metal element having a high melting point, such as tungsten (W) and molybdenum (Mo)) is considered. However, the resistance of these elements is very high compared with that of Al (see Table 1).
TABLE 1wiring materialresistivity [μΩcm]Al2W10~20Mo15~25
Therefore, when a screen size is enlarged, a wiring delay becomes a problem. In view of this, a method for decreasing a resistance by making wiring thicker is considered. However, when the width of wiring is enlarged, a degree of design freedom and an aperture ratio in a pixel portion may be lowered. Furthermore, when the film thickness of wiring is made larger, a short-circuit is likely to be caused at a portion where wiring crosses to each other three-dimensionally, and coverage is degraded at the step difference portion of the wiring.